1. Field of the Invention
The present invention relates to a multi-layer thin-film electrode used in a high-frequency band, such as microwave, sub-millimeter wave, or millimeter wavebands.
2. Description of the Related Art
Recently, electronic parts have become smaller in size. The size of high-frequency devices used in a high-frequency band, such as microwave, sub-millimeter wave, or millimeter wavebands is also reduced by employing materials having a high dielectric constant. However, when a smaller size is achieved by increasing the dielectric constant, the energy loss is disadvantageously raised in inverse proportion to the cubic root of volume. The energy loss of high-frequency devices can be roughly classified into a conductor loss due to skin effect and a dielectric loss due to dielectric materials. Recently, dielectric materials having a high dielectric constant with a low dielectric loss have been put to practical use. Therefore, the conductor loss is more dominant than the dielectric loss in determining unloaded-Q of a circuit.
Under the above circumstances, in a laid-open international application NO. WO 95/06336, the inventors of the present invention proposed a multi-layer thin-film electrode which can reduce the conductor loss in high-frequency bands. FIG. 4 is a perspective view of a 1/2 .lambda. line resonator composed of the conventional multi-layer thin-film electrode 200 indicated in the international application. The multi-layer thin-film electrode 200 is prepared as follows: a grounded conductor 11 is formed on the entire rear surface of a dielectric substrate 10; a band-shape thin conductor film 3a whose length is .lambda.g/2 (.lambda.g indicates wavelength in waveguide) in the longitudinal direction, is formed on the dielectric substrate 10; and then a thin dielectric film 30a-2, a thin conductor film 2a, a thin dielectric film 30a-1, and a thin conductor film 1a are laminated on the thin conductor film 3a in the given order to complete the multi-layer thin-film electrode 200 on the dielectric substrate 10.
As above-mentioned, a microstrip line (hereinafter referred to as "main transmission line") LN 10a for TEM mode is formed by the thin conductor film 3a, the grounded conductor 11, and the dielectric substrate 10 sandwiched between the thin conductor film 3a and the grounded conductor 11. Meanwhile, above the main transmission line LN10a, a sub transmission line for TEM mode is formed by sandwiching the thin dielectric film 30a-2 between one pair of thin conductor films 2a and 3a, and another sub transmission line for TEM mode is formed by sandwiching the thin dielectric film 30a-1 between one pair of thin conductor films 1a and 2a. According to a method disclosed in WO 95/06336, the conventional multi-layer thin-film electrode 200 is set up as follows:
(a) the thickness and the dielectric constants .di-elect cons..sub.s of the thin dielectric film 30a-1 and those of the thin dielectric film 30a-2 are set to predetermined values, respectively, so that the TEM wave transmitted through the main transmission line LN10a and the sub transmission lines, respectively, have substantially the same phase velocity; and PA1 (b) the thickness of the thin conductor film 2a and that of the thin conductor film 3a are set to predetermined values, respectively, which values are thinner than the skin depth at an operation frequency so that the electro-magnetic field of the main transmission line LN 10a and that of the sub transmission line adjacent thereto are coupled and the electro-magnetic fields of the sub transmission lines adjacent to each other are coupled.
Thus, the high-frequency energy flowing into the main transmission line LN 10a partially flows into the sub transmission lines so that high-frequency current flows through each of the thin conductor films 1a to 3a. The skin effect in the multi-layer thin-film electrode 200 is thereby largely suppressed at high frequencies.
A 1/2 .lambda. line resonator as shown in FIG. 4 can operate as a band-pass filter when connected to an external circuit via a conductor 12 for an input terminal and a conductor 13 for an output terminal, which conductors 12 and 13 are formed on the dielectric substrate 10.
However, such conventional multi-layer thin-film electrodes disadvantageously have low adhesive strength between the dielectric substrate and a thin conductor film adjacent thereto and between each thin dielectric film and a thin conductor film adjacent thereto, resulting in reduced reliability. In addition, when interlayer adhesive conductive films are provided for improving the adhesive strength between the thin dielectric substrate and a thin conductor film adjacent thereto and between each thin dielectric film and a thin conductor film adjacent thereto, skin effect cannot be satisfactorily suppressed.